"He just didn't meet the criteria for any further intervention," Santa Barbara County Sheriff Bill Brown said on CBS' "Face the Nation" on Sunday. "He was able to make a very convincing story that there was no problem, that he wasn't going to hurt himself or anyone else."
Like many other states, California has a law intended to identify and confine dangerously unstable people before they can do harm. It allows authorities to hold people in a mental hospital for up to 72 hours for observation.
To trigger it, there must be evidence a person is suicidal, intent on hurting others or so "gravely disabled" as to be unable to care for himself.
Police and medical personnel make tens of thousands of such welfare checks in California annually. In the year that ended June 2012, nearly 126,000 people were placed on temporary mental health holds in California.
In Rodger's case, it's not clear whether the law was too porous, if deputies were inadequately trained or if they simply weren't provided enough information to ferret out how deeply troubled Rodger had become.
For example, Rodger's mother knew at the time of the April 30 visit that her son had been posting bizarre videos on YouTube, yet police have said they were unaware of any such footage until after the rampage last Friday.
Rodger had also been in therapy for years, and it's not known what, if anything, authorities knew about his psychiatric care.
Ideally, officers making welfare checks should gather as much evidence as possible beforehand, including family statements and videos, said Risdon Slate, a professor of criminology at Florida Southern College who has trained law enforcement personnel to recognize the signs of mental illness.
But even if the deputies are well-trained, "a person with mental illness may be able to hold it together long enough" to avoid appearing suspicious, Slate said.
Rick Wall, a retired Los Angeles police captain who created the agency's procedures for responding to people with mental problems, said many tend to have some "leakage" in their behavior that can be a tipoff to what they are planning to do.
"In this case the leakage was like a sieve, there was so much stuff out there," Wall said. "People were hearing this, but no one was connecting the dots. No one was forwarding the information to where it could have been put together."
Law enforcement authorities dealing with such cases must also strike a balance between public safety and individual liberty and privacy.
In his written materials that surfaced after the shootings, Rodger said his weapons were stashed steps away inside his apartment, along with his blueprint to "exact revenge on my enemies."
But his room was never searched on the day deputies visited. His parents have said they were unaware he owned guns, and police would have been unable to search his apartment unless they obtained a warrant or believed there was an imminent threat.
California Senate President Pro Tem Darrell Steinberg suggested Tuesday that authorities should be required to determine if a person being assessed has bought guns, and additional steps could include talking to roommates, neighbors and relatives.
Still, it's not clear whether involuntarily committing Rodger would have averted the bloodshed. In many cases, people must be set free after the 72 hours are up.
"That's the debate. That's the issue: liberty versus forced treatment," said Tony Beliz, a retired deputy director of the Los Angeles County Department of Mental Health.
"After Columbine, after Sandy Hook, after things like this, the usual arguments come up. On the gun side, it's take all the guns away — the extremists say — or give everybody a gun," Beliz said.
It's the "same thing with the mental illness side — it's 'Make the laws tougher, hospitalize everybody, throw away the key.' That's great until it's your kid."